The accumulation of unwanted material in low flow regions of process equipment is a problem which adversely affects the chemical process industries on a daily basis. Unwanted materials may comprise by-products, residues, polymers, scale, dusts, corrosion products, precipitates, or other solids, liquids or vapors. Unwanted materials may restrict process flows, initiate undesirable side reactions (such as stress corrosion cracking), seed polymer growth, and render secondary process devices—such as valves and instrumentation—inoperable. The accumulation of unwanted material is costly in that it requires process downtime and manpower to remove these materials and to repair associated damage to the process equipment and piping that may have occurred. In addition, hazardous situations can result when safety equipment, such as pressure relief devices, are isolated from the process through the accumulation of unwanted materials in the process connections to which they are attached.
In the chemical process industries, one especially prevalent category of unwanted material accumulation is the formation of “condensation polymer”. Condensation polymer forms when vapor-phase monomers condense onto equipment surfaces in the absence of suitable polymerization inhibitors and then undergo polymerization. Monomers known to undergo condensation polymerization include but are not limited to (meth)acrylic acid and its esters, vinyl chloride, hydrogen cyanide, acrylonitrile, styrene, and other vinyl monomers.
One known method for combating condensation polymerization in the vapor spaces of process equipment such as storage tanks, reaction kettles and distillation columns, is to maintain the surface temperature of the equipment above the dew point of the monomer(s); jacketing, insulation and electric or steam tracing of process equipment has been shown to be relatively effective in “open-flow” regions, i.e. regions where the vapor-phase monomer can freely flow away from the heated surfaces. This approach is ineffective, however, in low flow regions such as process connections where the monomer vapors can stagnate and become trapped; this situation is further exacerbated when the process connection is a vessel nozzle, oriented vertically on the top head of the process equipment in question.
Addition of vapor-phase inhibitors has also been tried as a means for preventing condensation polymerization inside process equipment such as distillation columns. Examples of such inhibitors include SO2 in hydrogen cyanide service and NPH in (meth)acrylic acid service. Because low flow regions, such as nozzles, are essentially stagnant, however, vapor-phase inhibitors tend not to flow into them and unwanted material accumulation can proceed essentially unabated.
Another method for combating condensation polymerization in the vapor spaces of process equipment is through the use of internal spray devices. European Published Patent Application No. 1 044 957 A1 teaches the use of a “spraying and supplying means” placed inside a distillation column to spray liquid on interior surfaces of the distillation column, with the intent of inhibiting polymer formation. By design, however, this type of spray device is most effective in addressing polymer growth in large, open-flow regions, such as the top head o a distillation column or storage tank. This approach fails to adequately prevent the simultaneous accumulation of material in small, low-flow regions, such as the process connections on the top heads of tanks and columns.
In those cases where accumulations must also be prevented in small process connections, such as vessel nozzles, European Published Patent Application No. 1 044 957 A1 suggests that one or more spray nozzles may additionally be employed to specifically spray these connections. FIG. 10 of the present application illustrates such an embodiment wherein a spraying and supplying means is employed in combination with a top head connection to which a relief device (typically a rupture disk and relief valve assembly) is attached. The insertion of a single spray nozzle into the process connection from below is illustrated; in this configuration, a liquid spray would be direct upward into the stagnant process connection. The installation of the spray nozzle is mechanically complex as well as invasive to the vessel, requiring a separate vessel penetration (not shown) and the positioning of the line(s) inside the vessel for supplying the liquid to the spray nozzle. With this approach, the elevation and alignment of the spray nozzle itself, as well as the flow rate and spray pattern, are critical and, in practice, it is quite difficult to obtain the proper combination of these variables such that polymer accumulation is wholly prevented. While the effectiveness of such an arrangement could be improved through the use of multiple spray nozzle and an overwhelmingly large quantity of liquid spray, such an approach is costly and impractical in commercial operations. Additionally, the physical presence of the spray nozzle(s) and its supply line(s) create an undesirable obstruction in the process connection, thereby interfering with the free flow of material through the attached relief device, and these components themselves also create new surfaces for the accumulation of condensation polymer! In some instances the direct impingement of pressurized spray liquid on the relief device may also cause mechanical fatigue, thereby shortening the service life of the rupture disk. Because of these limitations, the proposed process for inhibiting polymerization is impractical when applied to the small, low flow regions associated with process connections, such as relief lines.
The use of spray rings of various designs is also known in the art as a means for cleaning the inside surfaces of sight glasses. Numerous patents have been issued for these devices and enhancements thereto, including U.S. Pat. No. 3,402,418, U.S. Pat. No. 4,158,508 and U.S. Pat. No. 4,541,277. Yet despite familiarity with the needs of the chemical process industries, those skilled in the art of designing sight glass spray rings have not anticipated the use of such devices for the prevention o unwanted material accumulation in low flow zones such as process connections.
In fact, despite these known methods, there remains a long-felt need to provide a simplified, reliable, inexpensive and effective means for preventing unwanted material accumulation in low flow regions, such as process connections. This need is especially great in the case where safety relief systems are attached to process connections comprising low flow regions; the accumulation of unwanted material in these nozzles restricting the free flow of material through the nozzle when the relief device is called upon to operate, thereby limiting the capacity of the relief device and creating an unsafe condition. The present invention overcomes the deficiencies of the prior art while meeting the needs of the chemical process industries.